Quantum mechanics observation questions?

Question

I watched a few videos on the "two-slit experiments". I won't go into much detail because you won't be able to answer this question unless you already know enough about it. Anyways, it seemed that when the results were not observed before they hit the wall, there would be a wave pattern. They didn't mention if it always resulted in the wave pattern.

Will the pattern be a wave EVERY time if the results are not observed at the slits?

My second question: Another video showed that if the results were gathered, but not viewed (the data was deleted without looking at the values), that they results would be a wave pattern (delayed observation). If they were viewed, the pattern would be two vertical lines.

Is there research that shows this happens EVERY time?

Finally, if the following two questions are both "yes", then does the same result happen if a non-human observer views the results? I'm not sure any animal could comprehend the meaning of the values, but I know that some animals can understand numbers by learning how to count.

Answer 1

Jack-

Good question.  Yes, the pattern of the slit experiment is always a "wave" interference pattern when the question "Through which slit did the photon (or other particle) travel?"

If you set up a detector to find the slit through which each photon traveled, the pattern from each slit is not superimposed to make the interference pattern.

The "best" explanation, I believe, is within Richard Feynman's three volume lecture series.

If the animal in your question can determine the difference between "pictures" then that animal will not "see" the same results in both experiments either.  There is no good way to "beat the system".

Please read the book that I suggest.  You will not be disappointed.

-Fred

Answer 2

Just to make it clear, the second video is wrong. No one has to look at the results. The only requirement is that an interaction occurs at one slit or the other. You can't change the pattern by looking at the equipment or not.

Answer 3

To your first question, yes, the pattern is a wave every time.

Your second video, as Jose said, is wrong.  If the data is collected, the system is measured and the wave pattern will not be there.  I would like to expand on this a little bit.  The reason you see the wave pattern is because of superposition.  The photon is in a superposition of two states.  The first state is the state of traveling through the first slit.  The second state is the stated of traveling through the second slit.  The wave pattern comes from interference between these two states.  But if you measure the state after the slit, you will collapse the superpsoition into just one state, either the 1st or 2nd.  Since the photon is no longer in both states, there is no longer interference between the states and the wave pattern goes away.

Superposition is notoriously fragile.  It can only exist for reasonable amounts of time in very very simple system.  When a superposition exists in a complicated system, it decays and collapses very very quickly.  A photon going through a slit is very very simple system (one of the simplest systems imaginable), so the superposition can last long enough to see the interference.  But a person, or a measuring device, consists of more than 10^23 atoms and is a very very complicated system.  So any superposition in a person would vanish almost instantly.  When you measure a double slit experiment you couple the simple photon system to you, a very complicated system.  So the new system is (you + photon), which forces the photon superposition to decay quickly.  If a camera measures a photon, it still couples the complicated camera system to the simple photon system, which decays the photon superposition, whether or not you then look at the camera results.

My guess is that you misunderstood the 2nd video.  There are ways that you could measure the photon with a device and then delete the measurement that would allow the superposition to persist.  But it is not as simple as burning the film.  You need to delete the measurement in a way that there is no way of getting the information out ever again.  So you might couple the photon to the outside world, but then you undo the coupling to re-insulate the photon from any external influence.  How to exactly do this is complicated and depends on how you are measuring.  It would almost certainly involve recombining the photons from slit one with the photons from slit two, so no one would know where the photon came from.  But in general, once a measurement is taken, unless it is 'undone' or 'untaken', you will not see the interference pattern.  And not looking at the results is not an example of undoing a measurement.

And for your final question, yes, if a zebra where to look at the results, since a zebra is a complicated system, it too would cause the photon interference pattern to vanish.